Fragile X Syndrome (FXS), as implied by its name, is associated with a fragile site expressed as an isochromatid gap in the metaphase chromosome at map position Xq 27.3. Fragile X syndrome is a genetic disorder caused by a mutation in the 5′-untranslated region of the fragile X mental retardation 1 (FMR1) gene, located on the X chromosome. The mutation that causes FXS is associated with a CGG repeat in the fragile X mental retardation gene FMR1. In most healthy individuals, the total number of CGG repeats ranges from less than 10 to 40, with an average of about 29. In fragile X syndrome, the CGG sequence is repeated from 200 to more than 1,000 times. When a subject has more than about 200 CGG repeats, the fragile X gene becomes hypermethylated, which silences the gene. As a result, fragile X mental retardation protein (FMRP) is not produced, or is produced at reduced level, and the subject displays manifestations of FXS.
Premutation expansions (55-200 CGG repeats) of the FMR1 gene are frequent in the general population, with estimated prevalences of 1 per 259 females and 1 per 812 males. Carriers of the premutation typically have normal IQ, although emotional problems such as anxiety are common. Older male carriers of the premutation (50 years and older) develop progressive intention tremor and ataxia. These movement disorders are frequently accompanied by progressive cognitive and behavioral difficulties, including memory loss, anxiety, and deficits of executive function, reclusive or irritable behavior, and dementia. This disorder has been designated fragile X-associated tremor/ataxia syndrome (FXTAS). Magnetic resonance imaging in subjects with FXTAS reveals increases in T2-weighted signal intensity in the middle cerebellar peduncles and adjacent cerebellar white matter.
FXS segregates as an X-linked dominant disorder with reduced penetrance. Either sex when carrying the fragile X mutation may exhibit intellectual disability, which is variable in severity. Children and adults with FXS have varying degrees of intellectual disability or learning disabilities and behavioral and emotional problems, including autistic-like features and tendencies. Young children with FXS often have delays in developmental milestones, such as learning how to sit, walk and talk. Affected children may have frequent tantrums, difficulties in paying attention, frequent seizures (e.g., temporal lobe seizures), are often highly anxious, easily overwhelmed, can have sensory hyperarousal disorder, gastrointestinal disorders, and may have speech problems and unusual behaviors, such as hand flapping and hand biting.
FXS can be diagnosed by an established genetic test performed on a sample (e.g., blood sample, buccal sample) from the subject. The test determines whether a mutation or premutation is present in the FMR1 gene of the subject based upon the number of CGG repeats.
Subjects with FXS can also have autism. About 5% of all children diagnosed with autism have a mutation in the FMR1 gene and also have fragile X syndrome (FXS). Autism spectrum disorder (ASD) is seen in approximately 30% of males and 20% of females with FXS, and an additional 30% of FXS individuals display autistic symptoms without having the ASD diagnosis. Although intellectual disability is a hallmark feature of FXS, subjects with FXS often display autistic features ranging from shyness, poor eye contact, and social anxiety in mild cases to hand flapping, hand biting and perseverative speech in the severely affected. Subjects with FXS display other symptoms associated with autism such as attention deficit and hyperactivity, seizures, hypersensitivity to sensory stimuli obsessive-compulsive behavior and altered gastrointestinal function. The FMR1 mutation prevents or greatly decreases expression of a single protein (FMRP). Brain development in the absence of FMRP is thought to give rise to the major symptoms of FXS.
In addition to core symptoms, children with FXS frequently have serious behavioral disturbances such as irritability, aggression and self-injurious behaviors. In a recent study of males with FXS (ages 8-24), self-injurious behavior was reported in 79%, and aggressive behavior in 75%, of subjects during a two month observation period.
Currently available treatment regimens for humans with FXS include, for example, behavioral modifications and treatment with a range of medications (not approved by FDA for the treatment of FXS) including antidepressant and antipsychotic drugs. Cognitive behavioral therapy has been used to improve language and socialization in individuals with FXS and autism. In recent years, pharmacological treatment with the atypical antipsychotic risperidone has been commonly employed to augment non-pharmacological approaches in the treatment of individuals with autism. A randomized placebo-controlled trial of risperidone in autistic children demonstrated significant improvement on the irritability subscale of the Aberrant Behavior Checklist and the Clinical Global Impressions-Improvement (McCracken, J. T., et al., N. Engl. J. Med. 347:314-321 (2002)). However, adverse events included weight gain, increased appetite, fatigue, drowsiness, dizziness, and drooling. Social isolation and communication were not improved by administration of risperidone and adverse side effects such as extrapyramidal symptoms and dyskinesias have been associated with risperidone use in autistic children. Since current treatment regimens are frequently not effective or may produce undesirable side-effects with long term use, particularly in the case of antipsychotic drugs, there is a need to develop new treatments.